Radiographic image diagnosis device

ABSTRACT

A correction table unit ( 5 ) contains correction tables for correcting output values of image data of the first image, the second image, . . . , the n-th image successively output from an X-ray plane detector ( 4 ) of a radiographic image diagnosis device and having different output rise characteristics. After correction of the image data, the second image data is subtracted from the first image data, . . . , and the n-th image data is subtracted from the first image data by a subtraction unit ( 8 ). As a result, a subtraction image, i.e., an angiographic image is displayed on display means ( 9 ). Since the affect of the different rise characteristics of each image data is eliminated, it is possible to obtain an angiographic image having a uniform background brightness and a preferable contrast.

FIELD OF THE INVENTION

The present invention relates to an X-ray diagnosis apparatus having anX ray flat panel detector and in particular, relates to an X-raydiagnosis apparatus which is provided with a function of correcting alow output characteristic at an initial stage of image taking by an Xray flat panel detector.

BACKGROUND ART

These days X-ray diagnosis apparatuses use as their X ray detectors an Xray flat panel detector. The X ray flat panel detector is constituted bya scintillator (for example, made of cesium iodide (CsI)) which convertsX ray transmitted through an object to be examined into light,photodiodes (for example, made by amorphous silicon (a-Si)) whichconverts the light output from the scintillator into electric charge andswitching elements such as, for example, thin film transistors (TFT)which read the electric charge, and, for example, has an active area of30×40 cm² and 7,400,000 pixels.

However, as referred to, for example, in R. L. Weisfield et al. “HighPerformance Amorphous Silicon Image Sensor for X-ray Diagnostic MedicalImaging Application” (SPIE vol. 3659, Medical Imaging 1999, part of theSPIE Conference on Physics of Medical Imaging, San Diego, Calif.,February 1999 pp 307-317), since the photo diodes in such X ray flatpanel detector are not saturated at the initial stage of the imagetaking, the output thereof shows lower in comparison with when the photodiodes are saturated. For this reason when the imaging is performedsuccessively from the initial stage thereof, the display brightnessshows dark at the initial stage and gradually becomes bright.

When an angiographic detection is performed with such X-ray diagnosisapparatus, at the same time when contrast medium is injected to theobject X ray is for the first time irradiated to the object, then X rayis successively irradiated to the object with a predetermined intervaland the image data at respective moments are successively output fromthe X ray detector. In this instance, by making use of a first imagedata which is output first as a mask image and by subtracting from imagedate after second ones, only a blood vessel image where the contrastmedium is injected is extracted and such as constriction of blood vesselis diagnosed from the image.

Namely, for the same X ray dose, the following tendency is observed thatthe signal value of the image data taken first which is used as the-maskimage shows the lowest value and the signal values taken second, third .. . , which are used as live images successively increase toward thesaturation. However, in the field of X ray detection, it is notpreferable in view of X ray exposure of an object to irradiate X ray tothe object prior to image taking irradiation.

FIG. 5 shows a build up output characteristic diagram of an X ray flatpanel detector wherein the abscissa is image number according to outputorder and the ordinate is relative height of the output signal. In FIG.5, symbols corresponding to letters a˜g show different X ray doses(a>b>c>d>e>f>g) irradiated to the same X ray flat panel detector. Theimage number according to output order shows image taking order by thesame X ray flat panel detector, in that first image, second image, thirdimage, . . . , tenth image.

Since the X ray flat panel detector shows the build up outputcharacteristic as shown in FIG. 5, the output values in the regionsother than the contrast medium of the image data taken first which isused as the mask image and of the image data taken second and thereafterwhich are used as live images are different, therefore, even if thesubtraction processing is performed, the regions other that the contrastmedium can not sometimes be sufficiently subtracted, as a result,difference in brightness level of the background portion in thesubtraction images which are used for the angiographic detection occursand the contrast of the subtraction images is reduced, which causes aproblem.

An object of the present invention is to provide an X-ray diagnosisapparatus which eliminates influences of inherent low outputcharacteristic, namely, delay in output build up of an X ray flat paneldetector in the X-ray diagnosis apparatus and permits to obtainsubtraction images, namely, angiographic images of desirable contrast.

Another object of the present invention is to provide an X-ray diagnosisapparatus which eliminates influences of inherent low outputcharacteristic of an X ray flat panel detector in the X-ray diagnosisapparatus and permits to obtain X ray images of uniform brightness whichfacilitates comparison observation.

DISCLOSURE OF THE INVENTION

A principle of the present invention is to match one of a build upoutput characteristic of, for example, a first image data value which isoutput from an X ray detector in an X-ray diagnosis apparatus and isused as a mask image and of a build up output characteristic of a secondimage data value and thereafter which is used as a live image with theother, and thereafter, subtraction processing of the both is performed,thereby, an influence due to different build up output characteristicsof the respective images is eliminated and desirable subtraction imagescan be obtained.

More specifically, in the present invention, for example, the outputdata value of the tenth image shown in FIG. 5 when the inherent initiallow output build up characteristic of the X ray flat panel detector issaturated is used as a reference and an output data value correctiontable for the respective first, second, . . . , nth images (n is aninteger more than 1) is prepared in advance and stores the same in amemory, and when taking X ray images and display the same, prior toexecuting the subtraction processing for obtaining subtraction images,at first, the image data successively output from the X ray flat paneldetector are corrected according to the output order by making use ofcorrection tables corresponding to the order which are prepared inadvance and stored in the memory, thereafter, by subtracting thecorrected mask image data value from the respective corrected live imagedata values, thus the subtraction images of desirable contrast areobtained.

Further, according to another aspect of the present invention, forexample, the output data value of the tenth image as shown in FIG. 5when the inherent initial low output build up characteristic of the Xray flat panel detector is saturated is used as a reference and anoutput data value correction table for the respective first, second, . .. , nth images (n is an integer more than 1) is prepared in advance andstores the same in a memory, and when taking X ray images and displaythe same, after correcting the respective image output data by makinguse of the correction tables and by outputting the corrected imageoutput data on a display, X ray images of uniform brightness whichfacilitates comparison observation are obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an outline structure representing anembodiment of an X-ray diagnosis apparatus according to the presentinvention;

FIG. 2 is a view for explaining characteristic of a first image data,which is output from an X ray flat panel detector 4 in FIG. 1 whereinthe abscissa is X ray doses and the ordinate is relative values ofoutput signal height corresponding to the doses;

FIG. 3 is a diagram of characteristic curve showing a correction tablestored in a correction table 5 in FIG. 1 for correcting a first imagedata output from an X ray flat panel detector 4 and used as a maskimage;

FIG. 4 is graphs showing output values on a certain one line of a firstimage data before and after correcting the same with the correctiontable as shown in FIG. 3; and

FIG. 5 is a diagram of characteristic curves showing relative values ofoutput signal height from a first image to a tenth image of the X rayflat panel detector in FIG. 1 using X ray doses as the parameter.

BEST MODES FOR CARRYING OUT THE INVENTION

Herein below, an embodiment of the present invention will be explainedwith reference to drawings.

FIG. 1 is a block diagram showing an outline structure representing anembodiment of an X-ray diagnosis apparatus with an X ray flat paneldetector according to the present invention.

As shown in FIG. 1, the X-ray diagnosis apparatus according to thepresent embodiment is provided with an X ray source 2 which irradiates Xray to an object 1, an X ray generation controller 3 which controls theirradiation of X ray from the X ray source 2 to the subject 1, an X rayflat panel detector 4 which detects X ray transmitted through thesubject 1 and outputs as an image signal, a correction table 5 having aplurality of correction tables which correspond to the output order ofimage data representing the image signals successively output from the Xray flat panel detector 4, a correction control circuit 6 which controlsthe X ray generation controller 3 and the X ray flat panel detector 4and selects from the correction table 5 a table corresponding to a imagedata representing the image signal output from the X ray flat paneldetector 4, an image memory 7 which stores the image signal output firstfrom the X ray flat panel detector 4 and corrected by the correctiontable 5 as a mask image data, a processor 8 which performs a subtractionprocess between the corrected image data successively output from the Xray flat panel detector 4 and corrected through the correction table 5and the mask image data stored in the image memory 7 and a display means9 which displays the image data subjected to the subtraction process bythe processor 8, in that subtraction images.

Now, an operation of the X-ray diagnosis apparatus according to thepresent embodiment will be explained.

At the same time when an injection of contrast medium to the subjectbegins, the correction control circuit 6 issues a command to the X raygeneration controller 3 to irradiate X ray from the X ray source 2 andfurther issues a command to fetch image data from the X ray flat paneldetector 4 in synchronism with the irradiated X ray. In response to thecommand from the correction control circuit 6, the image data outputfirst from the X ray flat panel detector 4 is sent to the correctiontable 5. When the correction table 5 judges that the image data sentfrom the X ray flat panel detector 4 is the image data output firstaccording to a signal from the correction control circuit 6, selects atable for correcting the first image data, corrects the image data valuebased on the correction table and stores the same in the image memory 7.

FIG. 2 is a characteristic curve diagram showing output characteristicof first image data output from the X ray flat panel detector 4 withrespect to transmitted doses, wherein the abscissa is X ray doses andthe ordinate is relative values of output signal. The plotted points inFIG. 2 correspond to relative output values with respect to seven X raydoses for first images, and based on these points, an approximationcurve is formed for preparing the correction table for the first image.The approximation curve is determined by making use of generally knownmethod, and the approximation curve as shown in FIG. 2 is calculatedaccording to logarithmic approximation.

FIG. 3 is a diagram of characteristic curve showing a correction tablefor correcting a first image data output from an X ray flat paneldetector 4 wherein the abscissa is input values (image data valuesbefore correction) and the ordinate is output values (image data valuesafter correction). After calculating correction amounts for therespective image data values according to the approximation curvedetermined in FIG. 2, the table for correcting first output image datafrom the X ray flat panel detector 4 is prepared. In FIG. 3 the table isset in such a manner that the lower the image data values are, thehigher the image data after correction are.

FIG. 4 is characteristic line diagram showing output values on a certainone line of a first output image data before correction and aftercorrection with the correction table as shown in FIG. 3 wherein theabscissa indicates positions on the one line and the ordinate indicatesoutput values, and (a) represents characteristic line diagram beforecorrection and (b) represents characteristic line diagram aftercorrection.

Through the use of the correction table as shown in FIG. 3, the firstimage output from the X ray flat panel detector 4 is corrected from theoutput value distribution represented by the characteristic line diagrambefore correction (a) as shown in FIG. 4 to the distribution representedby the characteristic line diagram after correction (b). As seen fromFIG. 4, it is understood that the output values at the portions wheredelay of output build up is significant due to non-saturation arecorrected greatly.

The corrected first output image data is once stored in the image memory7. Subsequently, in response to the command from the correction controlcircuit 6, when the second image data is output from the X ray flatpanel detector 4, the correction control circuit 6 selects from thetable 5 a table for correcting the second output image data to executethe correction. The correction table used in this instance is preparedin the same manner when the correction table for the first image wasprepared. The second output image data corrected by the correction table5 is subjected to subtraction process through the processor 8 by thecorrected first image data stored in the image memory 7, thereby,regions other than the region where the contrast medium is injected areerased, and an image in which the blood vessel image stands out wherethe contrast medium is injected is displayed on the display means 9 andbased on the displayed image, diagnosis is performed.

The variable components in the output characteristic of the image dataof third and thereafter output from the X ray flat panel detector 4 areremoved in the same manner as above, thus on the display means 9 bloodvessel images having desirable contrast and uniform backgroundbrightness are always displayed.

Further, in the above embodiment, although, an example was explained ofpreparing a correction table in which image data values (tenth outputimage in FIG. 5) when the output variation of the X ray flat paneldetector substantially disappears due to saturation of the capacitorportions of the photo diodes contained therein are used as a reference,the correction table can be prepared while using the first output imageas a reference and matching the output image thereafter to the firstoutput image data value.

Further, each of the respective correction tables for the first, second,. . . , nth image data stored in the correction table 5 can be preparedin plural number in advance based on several parameters such asirradiation X ray doses, frame rate representing number of photographingper unit time, gain of the X ray flat panel detector which is varieddepending on target photographing portions and image taking mode such ashigh image quality mode and high speed image collection mode, and stillfurther, the respective reference tables can be used while modifyingeach time based on the above parameters.

Still further, in the above, the subtraction process of second imagedata—first image data, third image data—first image data, . . . , nthimage data—first image data through the processor 8 has been explained,however, in the processor 8 subtraction process of n−(n−1), n−(n−2), . .. , n−2 can be executed with respect to the respective image data aftercorrection.

Still further, in the above embodiment, an example of applyingsubtraction process on the corrected images has been explained, however,without performing the subtraction in the processor 8 the correctedimages can be displayed as they are on the display means 9. Since thebrightness of the corrected images obtained in the above manner isuniform, images which facilitates comparison observation can be provided

Heretofore, the embodiments according to the present invention have beenexplained with reference to the drawings, however, the present inventiondoes not limited to the embodiments, but can be practiced by modifyingthe same within the gist of the present invention.

INDUSTRIAL APPLICABILITY

The X-ray diagnosis apparatus with the X ray flat panel detectoraccording to the present invention, in which X ray is irradiated to anobject in plurality of times in a predetermined interval, a plurality ofX ray images are taken and subtraction process is executed between theseplurality of X ray image data and which is useful for obtainingsubtraction images having desirable contrast and is, in particular,suitable for angiographic image detection.

1. An X-ray diagnosis apparatus comprising: an X ray source whichirradiates X ray to an object to be examined in a plurality of timeswith a predetermined time interval; an X ray flat panel detector whichdetects X ray each time transmitted through the subject and outputs thesame as image data; a correction table which stores a characteristic ofX ray dose versus output signal height of the X ray flat panel detectorwith respect to a first, second, third, . . . , nth (n is an integermore than 1) image data output from the X ray flat panel detector; and aprocessor which executes subtraction process of at least the secondimage data value—the first image data value, the third image datavalue—the first image data value, . . . , the nth image data value—thefirst image data value, wherein the subtraction process in the processoris executed after matching one of the characteristic of X ray doseversus output signal height of the image data to be subtracted and thecharacteristic of X ray dose versus output signal height of the imagedata to subtract to the other.
 2. An X-ray diagnosis apparatuscomprising: an X ray source which irradiates X ray to an object to beexamined in a plurality of times with a predetermined time interval; anX ray flat panel detector which detects X ray each time transmittedthrough the subject and outputs the same as image data; a correctiontable which stores tables for eliminating respective characteristicdifferences of X ray dose versus output signal height of the X ray flatpanel detector with respect to a first, second, third, . . . , nth (n isan integer more than 1)image data output from the X ray flat paneldetector; a processor which executes subtraction process of at least thesecond image data value—the first image data value, the third image datavalue—the first image data value, . . . , the nth image data value—thefirst image data value with respect to the respective image datacorrected through the correction table and outputs subtraction images;and a display which displays the output data from the processor asimages.
 3. An X-ray diagnosis apparatus according to claim 2, whereinthe respective correction tables are set in such a manner that for acomparatively low X ray doses a comparatively large correction is addedand for comparatively high X ray doses a comparatively small correctionis added.
 4. An X-ray diagnosis apparatus according to claim 2, whereinthe respective correction tables are modified by at least one of framerate, gain of the X ray flat panel detector and image taking mode.
 5. AnX-ray diagnosis apparatus according to claim 2, wherein the outputcorrection amount of the respective correction tables for the first,second, third, . . . , nth image data are set so as to decreasegradually.
 6. An X-ray diagnosis apparatus according to claim 2, whereinthe image data corrected by the respective correction tables aredirectly displayed on the display not through the processor.
 7. An X-raydiagnosis apparatus comprising: an X ray source which irradiates X rayto an object to be examined in a plurality of times with a predeterminedtime interval; an X ray flat panel detector which is disposed to facethe X ray source, detects every plurality of times X ray transmittedthrough the subject and outputs the same as image data; and displaymeans which displays the respective image data output every plurality oftimes from the X ray flat panel detector, further comprising a storagemeans which stores respective input X ray versus output signalcharacteristics of the X ray flat panel detector being associated withimage data output every plurality of times from the X ray flat paneldetector; correction means which reads out the input X ray versus outputsignal characteristics of the X ray flat panel detector for theplurality of times stored in the storage means and corrects the imagedata for a predetermined time among the plurality of times by the readout input X ray versus output signal characteristics of the X ray flatpanel detector for the plurality of times; and means for displaying theimage data corrected by the correction means on the display means andfor controlling the same.
 8. An X-ray diagnosis apparatus according toclaim 7, wherein the correction means corrects the image data obtainedat one time among the plurality of times based on the input X ray versusoutput signal characteristic of the X ray flat panel detector associatedwith the image data and the input X ray versus output signalcharacteristic of the X ray flat panel detector associated with theimage data obtained at another time.
 9. An X-ray diagnosis apparatusaccording to claim 7, wherein the correction means selects an input Xray versus output signal characteristic of the X ray flat panel detectorstored in the storage means in such a manner that for a comparativelylow X ray doses a comparatively large correction is added and forcomparatively high X ray doses a comparatively small correction isadded.
 10. An X-ray diagnosis apparatus according to claim 7, whereinthe respective input X ray versus output signal characteristics of the Xray flat panel detector stored in the storage means are modified by atleast one of frame rate, gain of the X ray flat panel detector and imagetaking mode.
 11. An X-ray diagnosis apparatus according to claim 7,wherein the respective input X ray versus output signal characteristicsof the X ray flat panel detector stored in the storage means are set insuch a manner that the output correction amount of the respectivecorrection tables with respect to the respective image data obtained forthe plurality of times decreases gradually.
 12. An X-ray diagnosisapparatus according to claim 7, further comprising means for performingsubtraction process between the image data obtained at one time amongthe plurality of times and the image data obtained at another time,wherein the display means displays the processed result performed by thesubtraction process means as images.